Seeing in Slo-Mo

 

Long exposures can blur moving subjects and portions of scenes into fascinating forms, revealing flows of motion and form that can’t be seen in an image made with a short exposure. All you need is a slow shutter speed, a sturdy camera support and your imagination.

Getting Long Exposure Times

Most digital and film SLRs let you set shutter speeds down to 30 seconds. For longer exposures, you can use the B (bulb) setting. On B, the camera shutter opens when you press the shutter button and stays open as long as you keep the shutter button fully depressed. To prevent finger fatigue, and to keep you from accidentally jiggling the camera, use a locking cable release to hold the shutter button down during long exposures.

Some cameras have a T (time) setting rather than B. On T, the shutter opens when you fully depress the shutter button and stays open until you press the shutter button again, saving you the bother of holding the shutter button down. In bright light, you won’t be able to stop the lens down enough to use very long exposure times: at ISO 100, the exposure duration for a frontlit sunny scene at ƒ/22 would be 1/50 sec. But there’s a way to get long exposure times in bright light: the neutral-density filter.

Neutral-density filters reduce the amount of light transmitted to the film or image sensor, without otherwise altering the light. ND filters come in a variety of strengths. The strongest are Kodak’s gelatin No. 96 ND 4.00, which cuts the light by 13 1/3 stops (www.kodak.com), Hoya’s NDx400, which provides nine stops of ND (www.thkphoto.com) and Singh-Ray’s Vari-ND, which provides strengths from two to eight stops of ND as you rotate it (www.singh-ray.com). A polarizing filter also reduces the amount of light reaching the film or image sensor, without altering its color, but only by 1.3 stops or thereabouts—not strong enough for daylight long exposures.

Tripod Or Handheld?

Long-exposure shooting is best done with the camera mounted on a sturdy tripod, especially the subject blurs discussed here. A cable release or wireless electronic trigger will keep you from jiggling the camera as you press the shutter button, adding to steadiness.

If your camera/lens has a built-in stabilizer, you can do pan blurs, zoom blurs and flash blurs handheld. I’ve had good results down to 1/20 sec. with focal lengths in the 200-300mm range using stabilizers built into lenses and cameras. But effective subject blurs generally require exposure times considerably longer than that. And if the nonmoving portions of such scenes are blurred, the image will look like a mistake rather than a good photograph, so use a tripod. With some stabilized lenses and cameras, you should switch the stabilizer off when using a tripod; with others, you should activate it. Check the instruction manual for your gear.

The Subject Blur: Long Exposure, Stationary Camera

Choose a scene that contains both stationary and moving elements, such as a waterfall, breakers on a rocky beach, waves of grain or flowers undulating in the wind, blowing snow or rain, whirlpools and eddies or freeway traffic at night. Put the camera on a tripod and shoot at a slow shutter speed. The stationary portions of the scene will be recorded sharply, while the moving elements will blur.

How slow a shutter speed should you use? That depends on the subject, how fast it’s moving, how far it is from the camera, the lens focal length and your personal preferences. Experiment with a range of exposure times the first time you try blurs with a given subject, and you’ll quickly learn from examining the photos which shutter speeds you prefer for that subject. For the subjects just mentioned, try exposure times from 1/15 sec. to 30 sec., and see which resulting images you prefer.
You can use very long exposures to make unwanted people disappear from landscape and architectural shots.
If you expose for several seconds to a couple of minutes, people moving through the scene won't be in any one place long enough to register on the film or image sensor.

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The Pan Blur: Long Exposure, Moving Camera

A great way to photograph a subject moving across the scene is to use a slow shutter speed and pan the camera to track the subject. The slow shutter speed will blur the background into "speed streaks" that emphasize the subject’s speed, while the pan keeps the subject reasonably sharp. Good subjects for this technique include racing cars, running wildlife and birds flying across terrestrial backgrounds (birds against a plain blue sky won’t show a blurred background, since the background sky is all one tone).

It takes some practice to be able to do pan blurs well. Good practice subjects include cars driving by, dogs frolicking, birds flying and kids playing. Stand facing the spot where you want to photograph the subject, turn at the waist toward the approaching subject, pick up the subject in the viewfinder, track the subject with the camera by rotating at the waist and trip the shutter as the subject arrives at the desired point. Be sure to follow through and keep tracking the subject as you shoot; if you stop as you trip the shutter, the subject will blur.

You also can pan the camera horizontally or vertically across a static scene, such as colorful flowers or a stand of trees, to produce interesting abstract renderings. Try this with the camera on a tripod and handheld for different effects.

As with stationary camera blurs, the exposure duration required to produce the best pan-blur effect depends on the subject, how fast it’s moving, how far it is from the camera, whether it’s moving toward the camera or across the frame, the lens focal length and your personal preferences. Again, try a variety of exposure times and see which work best for you with a given subject. I like exposure times of 1/4 to 1/60 sec. for living subjects, and 1/15 to 1/125 sec. for racing-car pans. For pans across static scenes, try 1/4, 1/2 and one sec.

Some image-stabilizing systems can be used for panned shots, while others treat the panning motion as something to be countered; with the latter, unsharp images will result. Check the manual for your lens or camera to see how the stabilizer should be set for panned shots. That said, the lens I use most often has two stabilizer modes, one for normal shooting and one for panning, and in nearly 10 years of use, I haven’t noticed a difference.

The Zoom Blur: Still Subject, Stationary Camera, Moving Lens

You can produce an interesting blur effect by zooming a zoom lens during a longish exposure. For this technique, I like exposure times in the 1- to 2-sec. range because they give me more time to control the zoom effect, but exposures as short as 1/30 sec. can work. Experiment a bit to see what works best for you. It’s easier to work with the camera on a tripod, but you can try some zoom blurs handheld as well (push-pull zooms are better than rotation zooms for handheld zoom blurs). Note that most compact cameras don’t allow you to zoom the lens during an exposure; this is an SLR project.

You can zoom from wide to telephoto or from telephoto to wide; each produces a different effect. If you pause for half the exposure duration and then zoom, you’ll get an identifiable image of the subject as well as the zoom streaks.

Zoom blurs work best with high-contrast subjects; flat scenes yield murky images. Backlit flowers make good subjects; frontlit meadows do not. I’ve found that 3x and 4x zooms work best for this technique. If you have one of the popular extreme-range zooms, you might try just using a portion of its focal-length range instead of the full range. Experiment with different exposure times and zoom rates until you find your sweet spot.

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The Flash Blur: Combining Flash And Ambient Light

With nearby moving subjects, you can combine flash with a slow shutter speed. The brief flash duration will sharply freeze an image of the nearby subject, while the slow shutter speed produces a blurred ghost image from the ambient light. On a windy day, you can use flash to "freeze" nearby foreground flowers while background blooms, beyond flash range, blur.

You also can combine flash with ambient light to capture both detail and a sense of frenzy in images of birds battling over territory or female birds (frequently seen in breeding season). The long exposure time will record motion blur, while the brief flash exposure will freeze a sharp image of the combatants.

Most TTL flash systems automatically balance the flash and background exposures, but many D-SLRs provide both ambient light and flash exposure compensation, so you can adjust the ratio to your liking. Using a stop or two of minus exposure compensation for the ambient light will darken the background, a dramatic albeit not natural effect.

If your subject is moving across the frame, try rear sync flash, available with most of today’s SLRs and higher-end flash units. Normal front sync will produce ghost-image speed streaks in front of the moving subject, while rear sync will produce speed streaks behind the moving subject, a more natural effect.

The Digital Advantage

Digital cameras offer a few huge advantages for experimenting with blur effects: You can check the effect on the camera's LCD monitor right after shooting the image, and the metadata records the exposure time and other data for each shot, saving you the trouble (and shooting delays) of keeping notes manually. When you review your images on your computer monitor, just check the metadata to see what exposure times produced the results you like best.

Reciprocity Failure

If you shoot your blur effects on film, you may encounter reciprocity failure, a loss of film speed that occurs at very long (and very short) exposure times and causes underexposure unless you give more exposure than a meter reading calls for. Because color films have three or four emulsion layers, each of which loses speed at a different rate, color shifts occur along with overall underexposure.

Each film has its own reciprocity characteristics. Reciprocity data for a given film (telling you how much exposure compensation is needed for various exposure times and what corrective filtration is required with color films) is sometimes packaged with the film; otherwise, you can request it from the film manufacturer or find it on the company’s Website. With most of today’s films, reciprocity failure doesn’t require correction until exposure times exceed 10 seconds (60 seconds with some films).

Digital SLRs are minimally affected (if at all) by the exposure and color effects of long-exposure reciprocity failure, but image noise may increase as exposure times grow longer. If you’re shooting long-exposure blur effects with a digital camera, you should activate its long-exposure noise-reduction feature. If you find image noise to be a problem in long-exposure images, you might want to invest in a good noise-reduction software product such as ASF’s Digital GEM (www.asf.com), Imagenomic’s Noiseware (www.imagenomic.com) or PictureCode’s Noise Ninja (www.picturecode.com).